An electric power steering apparatus (EPS) which provides a steering system of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a handle (a steering wheel) 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a steering angle sensor 14 for detecting a steering angle θr of the handle 1 and a torque sensor 10 for detecting a steering torque Th of the handle 1, and a motor 20 for assisting a steering force of the handle 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal IG is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist control on the basis of a steering torque Th detected by the torque sensor 10 and a vehicle speed Vs detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 by means of a voltage control command value Vref obtained by performing compensation or the like to the current command value. The steering angle θr is detected from the steering angle sensor 14. It is possible to obtain the steering angle from a rotational sensor which is connected to the motor 20.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vs from the CAN 40. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
The control unit 30 mainly comprises a CPU (Central Processing Unit) (including an MPU (Micro Processing Unit) and an MCU (Micro Controller Unit)), and general functions performed by programs within the CPU are, for example, shown in FIG. 2.
The control unit 30 will be described with reference to FIG. 2. The steering torque Th detected from the torque sensor 10 and the vehicle speed Vs detected from the vehicle speed sensor 12 (or from the CAN) are inputted into a current command value calculating section 31 which calculates a current command value Iref1. The current command value calculating section 31 calculates a current command value Iref1 which is a current control target value supplied to the motor 20, based on the inputted steering torque Th and the vehicle speed Vs using an assist map or the like. The current command value Iref1 is inputted into a current limiting section 33 via an adding section 32A. The current command value Iref3 whose maximum current is limited by an overheat protecting condition, is inputted into a subtracting section 32B. A deviation Iref4 (=Iref3−Im) which a fed-back motor current value Im is subtracted from the current command value Iref3 is calculated at the subtracting section 32B, and the deviation Iref4 is inputted into a proportional and integral (PI) control section 35 for improving characteristics in a steering operating. The voltage control command value Vref whose characteristics are improved at the PI-control section 35 is inputted into a pulse width modulation (PWM) control section 36. Further, the motor 20 is PWM-driven via an inverter 37 as a driving section. The motor current value Im is detected by a motor current detector 38 and is inputted into the subtracting section 32B for the feedback.
Further, the rotational sensor 21 such as a resolver is connected to the motor 20 and an actual steering angle θs is detected. A compensation signal CM from a compensating section 34 is added at the adding section 32A. A compensation of the system is performed by adding the compensation signal CM, and a convergence, an inertia characteristic and so on are improved. The compensating section 34 adds a self-aligning torque (SAT) 343 to an inertia 342 at an adding section 344. The adding result is further added with a convergence 341 at an adding section 345. The adding result at the adding section 345 is treated as the compensation signal CM.
In such an electric power steering apparatus, vehicles that have an automatic steering control mode (a steering angle control mode of a parking assist or the like) and a manual steering control mode (an assist control mode), and have a switching function between the automatic steering control mode and the manual steering control mode have emerged in recent years. In a case of realizing the automatic steering, generally, the vehicles have a steering angle control and an assist control independently, and have a configuration having a switching function between the outputs of the steering angle control and the assist control. In the steering angle control, a position speed control having an excellent performance in a response and an external disturbance suppression is used. The positional control is constituted by a proportional control (P-control), and the speed control is constituted by a proportional and integral control (PI-control) and so on.
The general electric power steering apparatus that comprises the functions of the steering angle control mode and the assist control mode, and has a function which the steering control mode is switched, will be described with reference to FIG. 3. A rotational sensor 151 such as a resolver for detecting a motor rotational angle θs is connected to the motor 150. The motor 150 is driving-controlled via a vehicle-side electronic control unit (ECU) 130 and an electric power steering (EPS)-side electronic control unit (ECU) 140. The vehicle-side ECU 130 comprises a switching command section 131 that outputs a switching command SW such as the steering angle control mode or the assist control mode, based on a button, a switch and so on which an intension of a driver is indicated, and a target steering angle generating section 132 that generates a target steering angle θt, based on signals such as a camera (an image) or a laser radar. The actual steering angle θr, which is detected by the steering angle sensor 14 provided with the column shaft (the steering shaft or the handle shaft), is inputted into a steering angle control section 200 in the EPS-side ECU 140 via the ECU 130.
The switching command section 131 outputs the switching command SW, based on a signal which identifies a switching to the steering angle control mode, for example, by using the button or the switch which is provided with a dashboard or the vicinity of the handle and indicate the intension of the driver, or a vehicle status signal which indicates a parking mode or the like by using a shift lever. The switching command SW is inputted into a switching section 142 in the EPS-side ECU 140. The target steering angle generating section 132 generates the target steering angle θt by using a known method, based on data such as the camera (the image) or the laser radar, and inputs the target steering angle θt into the steering angle control section 200 in the EPS-side ECU 140.
The EPS-side ECU 140 comprises an assist control section 141 that outputs an assist control command value Itref calculated based on the steering torque Th and the vehicle speed Vs, a steering angle control section 200 that calculates and outputs a steering angle control command value Imref for the steering angle control based on the target steering angle θt, the actual steering angle θr and a motor angle speed ωr, a switching section 142 that switches between the assist control command value Itref and the steering angle control command value Imref by means of the switching command SW, a current control/driving section 143 that driving-controls the motor 150 based on a motor current command value Iref (=Itref or Imref) from the switching section 142 and a motor angle speed calculating section 144 that obtains the motor speed based on the motor rotational angle θs from the rotational sensor 151 and calculates the actual angle speed ωr by using the motor speed and the gear ratio. The motor angle speed calculating section 144 comprises a low pass filter (LPF) that is disposed at a rear stage of a calculation corresponding to differential and reduces a high frequency noise.
As shown in FIG. 4, the steering angle control section 200 comprises a position control section 210 that outputs a steering angle speed command value ωc so that the actual steering angle θr follows the target steering angle θt, and a speed control section 220 that outputs a steering angle control command value Imref so that the actual steering angle speed ωr follows the steering angle speed command value ωc. The switching section 142 switches between the assist control mode (manual steering control) by the assist control section 141 and the steering angle control mode (position/speed control mode) by the steering angle control section 200, outputs the assist control command value Itref in the assist control mode and outputs the steering angle control command value Imref in the steering angle control mode.
As well, the actual angle speed and the motor angle speed have a relationship with a ratio of the reduction mechanism.
In the electric power steering apparatus having such a function, immediately switching the mode by using the switch or the like when the steering mode is switched, the driver feels uncomfortable because the motor current command value Iref is sharply changed and a handle becomes an unnatural behavior.